Organic electroluminescent (EL) devices or organic light emitting diodes (OLEDs) are electronic devices that emit light in response to an applied potential. Tang et al. in Applied Physics Letters 51, p913, 1987; Journal of Applied Physics, 65, p3610, 1989; and commonly assigned U.S. Pat. No. 4,769,292 demonstrated highly efficient OLEDs. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved. FIG. 1 illustrates schematically the cross-sectional view of a prior art bottom-emitting OLED. Prior art bottom-emitting OLED device 101 includes a substrate 10, a transparent bottom-electrode 12a, an organic EL medium structure 14, and a reflective metallic top-electrode 16R. The organic EL medium structure can include one or more sublayers including a hole-injecting layer (HIL) 14a (not shown), a hole-transporting layer (HTL) 14b, a light emitting layer (LEL) 14c, an electron-transporting layer (ETL) 14d, and an electron-injecting layer (EIL) 14e (not shown). In FIG. 1 the transparent bottom-electrode 12a is the anode and the reflective metallic top-electrode 16R is the cathode; but the reverse can also be the case and if so the order of the sublayers in the organic EL medium structure 14 is reversed.
The luminance output efficiency is an important figure of merit parameter of an OLED device. It determines how much current or power is needed to drive an OLED to deliver a desired level of light output. In addition, since the lifetime of an OLED device correlates inversely to the operating current, a higher output efficiency OLED device lasts longer at an identical light output level.
A method that has been studied to improve luminance output efficiency of an OLED device is to use the microcavity effect. OLED devices utilizing microcavity effect (microcavity OLED devices) have been disclosed in the prior art (U.S. Pat. Nos. 6,406,801 B1; 5,780,174, and JP 11-288786). In a microcavity OLED device the organic EL medium structure is disposed between two highly reflecting mirrors, one of which is light transmissive. The reflecting mirrors form a Fabry-Perot microcavity that strongly modifies the emission properties of the organic EL medium structure disposed in the microcavity. Emission near the wavelength corresponding to the resonance wavelength of the cavity is enhanced through the light transmissive mirror and those with other wavelengths are suppressed. The use of a microcavity in an OLED device has been shown to reduce the emission bandwidth and improve the color purity, or chromaticity, of emission (U.S. Pat. No. 6,326,224 B1). The microcavity also dramatically changes the angular distribution of the emission from an OLED device. There also have been suggestions that the luminance output could be enhanced by the use of a microcavity (Yokoyama, Science, Vol. 256, p66, 1992; Jordan et al. Appl. Phys. Lett. 69, p1997, 1996). In most the reported cases, however, at least one of the reflecting mirrors is a Quarter Wave Stack (QWS). A QWS is a multi-layer stack of alternating high index and low index dielectric thin-films, each one a quarter wavelength thick. It can be tuned to have high reflectance, low transmittance, and low absorption over a desired range of wavelengths.
FIG. 2 illustrates schematically the cross-sectional view of an exemplary prior art QWS based microcavity OLED device 102. QWS based microcavity OLED device 102 includes a substrate 10, a QWS reflecting mirror 18 as a light transmissive reflector, a transparent bottom-electrode 12a, an organic EL medium structure 14, and a reflective metallic top-electrode 16R. A typical QWS reflecting mirror 18 is of the form TiO2:SiO2:TiO2:SiO2:TiO2 with TiO2 n=2.45 and SiO2 n=1.5 as in Jordan et al. Appl. Phys. Lett. 69, p1997, 1996. Thickness of each material is 56 nm and 92 nm, respectively, corresponding to quarter wavelength for green emission at 550 nm. In operation only a narrow band light centered at the resonance wavelength of 550 nm is emitted through the QWS layer out of the microcavity OLED device.
A QWS is complicated in structure and expensive to fabricate. The resonance bandwidth is extremely narrow and, as a result, even though a microcavity based on a QWS is capable of greatly increasing the emission peak height at the resonance wavelength, the total luminance integrated over wavelength is much less improved and can actually decrease over a similar device without the microcavity. In addition, the dielectric layers are not electrically conductive. To form a functional OLED device, a separate transparent conductive electrode layer needs to be disposed between the QWS and the organic layers. This added conductive electrode layer further complicates the structure. If a transparent conductive oxide is used as the conductive electrode, the electrical conductance is limited and can be inadequate for many devices especially those having large areas. If a thin metal film is used, the cavity structure is much more complicated and device performance can be compromised. QWS-based microcavity OLED devices are therefore not suitable for practical microcavity OLED device applications.
It is generally believed, however, that a QWS constructed of non-absorbing dielectric materials is necessary in achieving useful microcavity effects. Yokoyama, Science, Vol. 256, p66, 1992 specifically recommended the use of a QWS instead of metallic mirrors. Published attempts to replace the QWS with the more practical metal mirrors have not been very successful. Berggren et al. in Synthetic Metals 76, p121, 1996 studied a PLED using an Al opaque mirror and a Ca—Al light transmissive mirror to construct a microcavity. Although some bandwidth narrowing was observed suggesting a microcavity effect, the external quantum efficiency of the device with microcavity was a factor of three less than a similar device without a microcavity. Takada et al. in Appl. Phys. Lett. 63, p2032, 1993 constructed a microcavity OLED device using a light transmissive (36 nm) Ag cathode and a 250 nm MgAg opaque anode. Although angular distribution change and emission bandwidth reduction were observed, the emission intensity was significantly reduced compared with a non-cavity case. The authors concluded that the combination of emission dyes with broad emission spectra and a simple planar cavity was not satisfactory for the confinement of light in the microcavity, and encouraged development of new cavity structures. Jean et al. in Appl. Phys. Lett., Vol. 81, p1717, 2002 studied an OLED structure using a 100 nm Al as the opaque anode and a 30 nm Al as the light transmissive cathode to construct a microcavity structure. Although a strong microcavity effect caused emission bandwidth narrowing and a strong angular dependence was observed, no improvement in emission output efficiency was suggested. In fact judging from the extremely narrow emission bandwidth of the devices, the emission output efficiency was most likely decreased. EP 1 154 676 A1 disclosed an organic EL device having a bottom-electrode of a light reflective material, an organic light emitting layer, a light transmissive reflection layer, and a top-electrode of a transparent material forming a cavity structure. The objective was to achieve an OLED device with sufficient color reproduction range over a wide viewing angle. The objective was achieved by reducing the microcavity effect to achieve a large emission bandwidth. Although it alleged that multiple reflection enhances resonance wavelength emission, no actual or simulated data supported the suggestion. All examples used a Cr reflective anode. Our studies showed that little luminance enhancement is achievable when an anode with reflectivity as low as Cr is used.
Lu et al. in Appl. Phys. Lett., Vol. 81, p3921, 2002 described top-emitting OLED devices that the authors alleged to have performance enhanced by microcavity effects. However, their performance data showed very little angular dependence characteristic of microcavities. Although no spectral data were shown, the similarity in color coordinates between their non-cavity bottom-emitting structure and microcavity top-emitting structure suggests that the bandwidth narrowing effect expected in microcavity OLED devices is most likely absent as well. Indeed, our model calculations confirm that their structure should not produce a significant microcavity effect. Thus, the observed emission enhancement is most likely a result of normal modest optical interference effects typically seen in non-microcavity OLED devices. The magnitude of the emission enhancement is very small and the color quality improvement is absent. The authors also suggested that the best efficiency is achieved by using a high reflectivity anode and a transparent cathode, the latter being clearly contrary to the teaching of the present invention. H. Riel, S. Karg, T. Beierlein, B. Ruhstaller, and W. Rieb, Appl. Phys. Lett., Vol. 82, p466, 2003 reported a microcavity OLED device using a phosphorescent emitter and a dielectric layer over the semitransparent metal electrode. Although the device efficiency was much improved compared to similar microcavity devices without the dielectric layer, the extent of improvement over the corresponding non-microcavity device is not clear.